Polyisocyanate Production System and Gas Treatment Apparatus

ABSTRACT

A polyisocyanate production system is provided that can stably produce chlorine from hydrogen chloride produced secondarily while reacting stably between carbonyl chloride and polyamine and can perform an effective treatment of the hydrochloric gas produced secondarily. A hydrochloric gas control unit  32  controls a flow-rate control valve  23  to keep constant an amount of hydrogen chloride supplied from a hydrogen chloride purifying tank  4  to a hydrogen chloride oxidation reactor  6  via a second hydrochloric-gas connection line  11  to be constant, and also controls a pressure control valve  22  based on an inner pressure of the hydrogen chloride purifying tank  4  input from a pressure sensor  25  to discharge the hydrochloric gas from the hydrogen chloride purifying tank  4  to the hydrogen chloride absorbing column  5  via a first hydrochloric-gas connection line  10 , so as to keep an inner pressure of the hydrogen chloride purifying tank  4  to be constant.

TECHNICAL FIELD

The present invention relates to a polyisocyanate production system forproducing polyisocyanate used as a raw material of polyurethane, and toa gas treatment apparatus for treating gas by bringing the gas intocontact with a treatment liquid.

BACKGROUND ART

Polyisocyanate used as a raw material of polyurethane is industriallyproduced by reacting carbonyl chloride with polyamine for isocyanatereaction.

In this isocyanate reaction, corresponding polyisocyanate is producedfrom polyamine and hydrochloric gas is produced secondarily.

A production method for producing chlorine industrially by oxidizing thehydrochloric gas thus produced secondarily is known (cf. PatentDocuments 1 and 2 listed below, for example).

A plant for manufacturing chemicals is provided with a gas treatmentapparatus for detoxifying e.g. a harmful gas produced in a chemicalprocess. This gas treatment apparatus comprises, for example, a fillingcolumn, a spray column, and a scrubber, and the one used for treatingthe harmful gas is sometimes called a detoxification column.

A detoxification column shown in FIG. 4 is known as an example of thegas treatment apparatus (cf. Patent Document 3 list below, for example).

This detoxification column 71 includes a treatment tank 72, a storagetank 73, and a pump 74. The treatment tank 72 has a gas-liquid contactchamber 75 in which a packed material is packed to improve efficiency ofgas-liquid contact and also has showers 76 arranged over the gas-liquidcontact chamber 75. A bottom of the treatment tank 72, the storage tank73, the pump 74, and the showers 76 are connected via a circulation line77.

A treatment liquid for detoxifying the harmful gas is stored in thestorage tank 73. The treatment liquid is circulated in the followingsequence: The treatment liquid is first pumped up by the pump 74 upwardthrough the circulation line 77, then, sprayed from the showers 76 intothe gas-liquid contact chamber 75 of the treatment tank 72. Afterpassing through the gas-liquid contact chamber 75, the treatment liquidflows back to the storage tank 73 from the bottom of the treatment tank72.

On the other hand, a harmful gas is supplied to the treatment tank 72 insuch a manner as to flow upward from the bottom of the gas-liquidcontact chamber 75 so that the harmful gas contacts the treatment liquidsprayed from the showers 76 in the vertically opposite direction for aneffective gas-liquid contact, so that the harmful gas is detoxified.Then, the resultant gas is discharged from the treatment tank 72 to theatmosphere.

In Patent Document 3 listed below, a carbonyl-chloride-containing gas asthe harmful gas is detoxified by this detoxification column using asodium hydroxide liquid solution as the treatment liquid.

Document 1: Japanese Laid-open (Unexamined) Patent Publication No. Sho62-275001,

Patent Document 2: Japanese Laid-open (Unexamined) Patent PublicationNo. 2000-272906, and

Patent Document 3: Japanese Laid-open (Unexamined) Patent PublicationNo. Hei 6-319946.

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

When a production of polyisocyanate is attempted by the process thatcarbonyl chloride and polyamine undergo isocyanate reaction; thehydrochloric gas produced secondarily in the isocyanate reaction isoxidized by oxygen to thereby produce chlorine; the chlorine obtainedand carbon monoxide are reacted to produce carbonyl chloride; and thenthe carbonyl chloride obtained with the polyamine undergoes theisocyanate reaction, it is found that when the isocyanate reactionvaries for various reasons, an amount of hydrochloric gas producedsecondarily may vary. Then, when the amount of hydrochloric gas producedsecondarily varies, an amount of hydrochloric gas supplied in theoxidation process of oxidizing the hydrochloric gas varies.

To ensure an industrially steady production of chlorine, steady supplyof hydrochloric gas produced secondarily to a hydrochloric gas oxidationtank is required.

Also, to ensure a stable reaction between carbonyl chloride andpolyamine in an isocyanate producing reactor, an inner pressure of theisocyanate producing reactor must be kept constant.

Accordingly it is desired that even with variations of an isocyanateproducing reactor produced secondarily from the isocyanate producingreactor, the hydrochloric gas is treated effectively in consideration ofthese two factors described above.

In addition, when a trouble occurs in the hydrochloric gas oxidationtank for oxidizing the hydrochloric gas produced secondarily, thetreatment of the hydrochloric gas produced secondarily is troubled.

Further, when a trouble occurs, for example, in the pump 74 of thedetoxification column 71, the treatment liquid through the circulationline 77 is not circulated. As a result of this, the efficiency of thegas-liquid contact of the harmful gas with the treatment liquid in thetreatment tank 72 is reduced. Furthermore, when the pumping of thetreatment liquid is interrupted, the treatment liquid remaining in thegas-liquid contact chamber 75 may be consumed to cause a possibleproblem that the harmful gas cannot be detoxified.

It is an object of the present invention to provide a polyisocyanateproduction system that can stably produce chlorine from hydrogenchloride produced secondarily while ensuring stable reaction betweencarbonyl chloride and polyamine and can also provide effective treatmentof the hydrochloric gas produced secondarily.

It is another object of the present invention to provide apolyisocyanate production system that can provide effective treatment ofthe hydrogen chloride even when a trouble occurs in a chorine productionmeans for producing chlorine from the hydrogen chloride producedsecondarily.

It is a further another object of the present invention to provide a gastreatment apparatus that can supply a treatment liquid to the inside ofa gas-liquid contact chamber without any need of particular powersource.

Means for Solving the Problem

The present invention provides a polyisocyanate production systemcomprising a polyisocyanate producing means for producing polyisocyanateby reacting carbonyl chloride with polyamine, a hydrogen chloridepurifying means to which hydrogen chloride produced secondarily in thepolyisocyanate producing means is supplied to purify hydrogen chloride,a chlorine producing means to which the hydrogen chloride purified inthe hydrogen chloride purifying means is supplied and in which thehydrogen chloride is oxidized to produce the chlorine, a hydrochloricacid producing means to which the hydrogen chloride purified in thehydrogen chloride purifying means is supplied and in which the hydrogenchloride is absorbed in water to produce a hydrochloric acid, a firstadjusting means for adjusting an amount of hydrogen chloride suppliedfrom the hydrogen chloride purifying means to the hydrochloric acidproducing means, a second adjusting means for adjusting an amount ofhydrogen chloride supplied from the hydrogen chloride purifying means tothe chlorine producing means, and a control means for controlling thesecond adjusting means so that an amount of hydrogen chloride suppliedfrom the hydrogen chloride purifying means to the chlorine producingmeans is kept constant and controlling the first adjusting means so thatan inner pressure of the hydrogen chloride purifying means is constant.

According to this polyisocyanate production system, the control meanscontrols the second adjusting means to keep an amount of hydrogenchloride supplied from the hydrogen chloride purifying means to thechlorine producing means to be constant and also controls the firstadjusting means to adjust an amount of hydrogen chloride supplied fromthe hydrogen chloride purifying means to the chlorine producing means,so as to keep a pressure of an inside of the hydrogen chloride purifyingmeans constant. This provides the result that the hydrogen chloride canbe steadily supplied to the chlorine producing means, while the pressureof the inside of the hydrogen chloride purifying means can be keptconstant by supplying surplus hydrogen chloride to the hydrochloric acidproducing means from the hydrogen chloride purifying means.

As a result of this, chlorine can be steadily produced from the hydrogenchloride produced secondarily, while the inner pressure of the hydrogenchloride purifying means and thus the pressure of the inside of thepolyisocyanate producing means can be kept constant. This ensures astable reaction between carbonyl chloride and polyamine and enables aneffective treatment of the hydrochloric gas produced secondarily.

In this polyisocyanate production system, it is preferable thatunoxidized hydrogen chloride and hydrochloric acid in the chlorineproducing means are supplied to the hydrochloric acid producing means.

When unoxidized hydrogen chloride in the chlorine producing means andhydrochloric acid produced in the chlorine producing means are suppliedto the hydrochloric acid producing means without being discharged, thehydrochloric acid can be produced more effectively for effectiveutilization of the surplus hydrogen chloride.

In this polyisocyanate production system, it is preferable that thehydrochloric acid producing means is provided with ahydrochloric-acid-concentration adjusting means for adjusting aconcentration of the hydrochloric acid produced.

When a concentration of the hydrochloric acid produced is adjusted bythe hydrochloric-acid-concentration adjusting means, a hydrochloric acidof stable quality can be produced.

Further, the present invention provides a polyisocyanate productionsystem comprising a polyisocyanate producing means for producingpolyisocyanate by reacting carbonyl chloride with polyamine, a hydrogenchloride purifying means, connected to the polyisocyanate producingmeans for purifying the hydrogen chloride produced secondarily in thepolyisocyanate producing means, a chlorine producing means connected tothe hydrogen chloride purifying means for producing chlorine byoxidizing the hydrogen chloride purified in the hydrogen chloridepurifying means, a first detoxifying treatment means connected to thehydrogen chloride purifying means in parallel with respect to thechlorine producing means, for detoxifying the hydrogen chloridedischarged from the hydrogen chloride purifying means, a seconddetoxifying treatment means selectively connected to the hydrogenchloride purifying means with respect to the chlorine producing means,for detoxifying the hydrogen chloride discharged from the hydrogenchloride purifying means, an abnormality detecting means for detectingan abnormality of the hydrogen chloride purifying means, and aconnection switching means which connects the chlorine producing meanswith the hydrogen chloride purifying means when an abnormality is notdetected by the abnormality detecting means and connects the seconddetoxifying treatment means with the hydrogen chloride purifying meanswhen an abnormality is detected by the abnormality detecting means.

According to this polyisocyanate production system, when an abnormalityof the hydrogen chloride purifying means is not detected by theabnormality detecting means, the hydrogen chloride purified by thehydrogen chloride purifying means is supplied to the chlorine producingmeans to produce chlorine from the hydrogen chloride supplied in thechlorine producing means and is discharged to the first detoxifyingtreatment means to detoxify the hydrogen chloride supplied in the firsttreatment means.

On the other hand, when following an abnormality being in the chlorineproducing means, an abnormality of the hydrogen chloride purifying meansis detected by the abnormality detecting means, the connection switchingmeans switches the connection between the hydrogen chloride purifyingmeans and the chlorine producing means to the connection between thehydrogen chloride purifying means and the second detoxifying treatmentmeans according to a treating capability of the first detoxifyingtreatment means. Then, the hydrogen chloride supplied to the chlorineproducing means in the interim is discharged to the second detoxifyingtreatment means to detoxify the hydrogen chloride by the seconddetoxifying treatment means.

As a result of this, when the chlorine producing means is normal, thehydrogen chloride is steadily supplied to the chlorine producing means,while surplus hydrogen chloride is detoxified in the first detoxifyingtreatment means. On the other hand, when a trouble occurs in thechlorine producing means, the hydrogen chloride supplied to the chlorineproducing means in the interim is detoxified in the second detoxifyingtreatment means depending on an amount of the surplus hydrogen chloridewhich exceeds a treating capability of the first detoxifying treatmentmeans. This can achieve an effective treatment of the hydrogen chloride.

In this polyisocyanate production system, it is preferable that theconnection switching means comprises a first opening and closing meansfor connecting the chlorine producing means to connect to or disconnectfrom the hydrogen chloride purifying means, a second opening and closingmeans for connecting the second detoxifying treatment means to ordisconnecting from the hydrogen chloride purifying means, and a controlmeans for controlling the first opening and closing means and the secondopening and closing means, wherein when an abnormality is not detectedby the abnormality detecting means, the control means controls the firstopening and closing means to connect the chlorine producing means withthe hydrogen chloride purifying means and also controls the secondopening and closing means to disconnect the second detoxifying treatmentmeans from the hydrogen chloride purifying means, while on the otherhand, when an abnormality is detected by the abnormality detectingmeans, the control means controls the first opening and closing means todisconnect the chlorine producing means from the hydrogen chloridepurifying means or to reduce the hydrogen chloride purified in thehydrogen chloride purifying means rapidly in amount supplied to thechlorine producing means and also controls the second opening andclosing means to connect the second detoxifying treatment means with thehydrogen chloride purifying means according to a treating capability ofthe first detoxifying treatment means.

According to this polyisocyanate production system, when an abnormalityis not detected by the abnormality detecting means, the control meanscontrols the first opening and closing means to connect the chlorineproducing means with the hydrogen chloride purifying means and alsocontrols the second opening and closing means to disconnect the seconddetoxifying treatment means from the hydrogen chloride purifying means,while on the other hand, when an abnormality is detected by theabnormality detecting means, the control means controls the firstopening and closing means to disconnect the chlorine producing meansfrom the hydrogen chloride purifying means or to reduce the hydrogenchloride purified in the hydrogen chloride purifying means rapidly in anamount supplied to the chlorine producing means and also controls thesecond opening and closing means to connect the second detoxifyingtreatment means with the hydrogen chloride purifying means according toa treating capability of the first detoxifying treatment means. Thisprovides the result that when a trouble occurs in the chlorine producingmeans, hydrogen chloride supplied to the chlorine producing means in theinterim is surely discharged to the second detoxifying treatment meansto be detoxified depending on an amount of the surplus hydrogen chloridewhich exceeds a treating capability of the first detoxifying treatmentmeans.

In this polyisocyanate production system, it is preferable that thefirst detoxifying treatment means is a hydrochloric acid producing meansfor producing hydrochloric acid by absorbing the hydrogen chloride inwater.

When the first detoxifying treatment means comprises a hydrochloric acidproducing means, surplus hydrogen chloride can be detoxified, whilehydrochloric acid can be produced from the surplus hydrogen chloride forrecycling purpose. This produces the result that an effectiveutilization of the surplus hydrogen chloride can be made.

In this polyisocyanate production system, it is preferable that theunoxidized hydrogen chloride and the hydrochloric acid in the chlorineproducing means are supplied to the hydrochloric acid producing means.

When the unoxidized hydrogen chloride in the chlorine producing means orthe hydrochloric acid produced in the chlorine producing means issupplied to the hydrochloric acid producing means without beingdischarged therefrom, hydrochloric acid can be produced moreeffectively, thus providing the result that an effective utilization ofsurplus hydrogen chloride can be made.

It is preferable that this polyisocyanate production system comprises afirst adjusting means for adjusting an amount of hydrogen chloridesupplied from the hydrogen chloride purifying means to the hydrochloricacid producing means, a second adjusting means for adjusting an amountof hydrogen chloride supplied from the hydrogen chloride purifying meansto the chlorine producing means, and a control means for controlling thesecond adjusting means so that an amount of hydrogen chloride suppliedfrom the hydrogen chloride purifying means to the chlorine producingmeans is kept constant and controlling the first adjusting means so thatan inner pressure of the hydrogen chloride purifying means is keptconstant.

When the control means controls the second adjusting means so that anamount of hydrogen chloride supplied from the hydrogen chloridepurifying means to the chlorine producing means is kept constant andcontrols the first adjusting means to adjust an amount of hydrogenchloride supplied from the hydrogen chloride purifying means to thechlorine producing means so as to keep the inner pressure of thehydrogen chloride purifying means constant, the hydrogen chloride can besteadily supplied to the chlorine producing means, while surplushydrogen chloride is supplied from the hydrogen chloride purifying meansto the hydrochloric acid producing means, whereby the inner pressure ofthe hydrogen chloride purifying means is kept constant.

This produces the result that the chlorine can be steadily produced fromthe hydrogen chloride produced secondarily, while the inner pressure ofthe hydrogen chloride purifying means and thus the inner pressure of thepolyisocyanate producing means can be kept constant. This allows astable reaction between carbonyl chloride and polyamine to ensure aneffective treatment of the hydrogen chloride produced secondarily.

Further, the present invention provides a gas treatment apparatus fortreating a gas by bringing the gas into contact with a treatment liquid,which comprises a gas-liquid contact chamber for a gas-liquid contactbetween the gas and the treatment liquid, a storage chamber, locatedover the gas-liquid contact chamber, for storing the treatment liquid,and a treatment liquid supplying means for supplying the treatmentliquid stored in the storage chamber to an inside of the gas-liquidcontact chamber via a gravity-drop.

According to this gas treatment apparatus, the treatment liquid issupplied with a gravity drop from the storage chamber located over thegas-liquid contact chamber to the interior of the gas-liquid contactchamber by the treatment liquid supplying means without any specificpower source. Even when some trouble occurs in the power source used forsupplying the treatment liquid to the storage chamber, the treatmentliquid is steadily supplied to the interior of the gas-liquid contactchamber by the treatment liquid supplying means until the treatmentliquid in the storage tank runs out.

As a result of this, even when the supply of the treatment liquid to thegas-liquid contact chamber is interrupted due to some trouble in thepower source, the gas-liquid contact between the treatment liquid andthe gas in the gas-liquid contact chamber can be continued and also thetreatment of the gas can be continued, thus providing furtherimprovement in safety.

Preferably, the gas treatment apparatus of the present inventioncomprises a plurality of the gas-liquid contact chambers, and a gaspassage path for passing the gas in series through the respectivegas-liquid contact chambers, and the treatment liquid supplying meanssupplies the treatment liquid to the gas-liquid contact chamber locatedat least at a most downstream side with respect to a flowing directionof the gas flowing along the gas passage path.

When there are provided a plurality of gas-liquid contact chambers andthe gas passage path for passing the gas in series through thosegas-liquid contact chambers, a treatment of the gas can be carried outmultistage-wise and continuously, thus achieving an effective treatmentof the gas.

Further, since the treatment liquid supplying means supplies thetreatment liquid at least to the gas-liquid contact chamber located at amost downstream side with respect to a flowing direction of the gasflowing along the gas passage path even with providing of the pluralityof gas-liquid contact chambers, even when the supply of the treatmentliquid to the gas-liquid contact chambers is interrupted due to sometrouble in the power source, the gas-liquid contact between thetreatment liquid and the gas can be continued at least in the gas-liquidcontact chamber located at the most downstream side, thus providing afurther improvement in safety.

Preferably, the gas treatment apparatus of the present invention furthercomprises a treatment liquid back-flow means for returning the treatmentliquid discharged from the gas-liquid contact chamber to the storagechamber.

When the treatment liquid returns via the treatment liquid back-flowmeans, a waste of the treatment liquid can be minimized and runningcosts can be reduced.

The gas treatment apparatus of the present invention is preferably usedas a gas treatment apparatus for detoxifying thecarbonyl-chloride-containing gas in which the gas is acarbonyl-chloride-containing gas, and the treatment liquid is analkaline aqueous solution.

Also, the gas treatment apparatus of the present invention is preferablyused as a gas treatment apparatus using a hydrogen-chloride-containinggas as the gas and water or alkaline aqueous solution as the treatmentliquid used for detoxifying the hydrogen-chloride-containing gas.

Further, the gas treatment apparatus of the present invention ispreferably used as a gas treatment apparatus using a combination ofammonia- or alkylamine-containing gas and water or an acid aqueoussolution; a Sox- or NOx-containing gas and water or an alkaline aqueoussolution; a volatile-organic-compound-containing gas and an organicsolvent, etc. as a combination of the gas and the treatment liquid, forremoval (detoxification), absorption and collection of the gas.

EFFECT OF THE INVENTION

According to the polyisocyanate production system of the presentinvention, chlorine can be steadily produced from hydrogen chlorideproduced secondarily, while the inner pressure of the polyisocyanateproducing means can be kept constant. This ensures a stable reactionbetween carbonyl chloride and polyamine and an effective treatment ofthe hydrochloric gas produced secondarily.

According to the polyisocyanate production system of the presentinvention, when the chlorine producing means is normal, the hydrogenchloride is steadily supplied to the chlorine producing means, whilesurplus hydrogen chloride is detoxified in the first detoxifyingtreatment means. On the other hand, when some trouble occurs in thechlorine producing means, the hydrogen chloride supplied to the chlorineproducing means in the interim is detoxified in the second detoxifyingtreatment means depending on an amount of the surplus hydrogen chloridewhich exceeds a treating capability of the first detoxifying treatmentmeans. This provides the result that regardless of the treatingcapability of the first detoxifying treatment means, the hydrogenchloride can be detoxified, thus achieving an effective treatment of thehydrogen chloride.

According to the polyisocyanate production system of the presentinvention, even when the supply of the treatment liquid to thegas-liquid contact chamber is interrupted due to some trouble in thepower source, the gas-liquid contact between the treatment liquid andthe gas in the gas-liquid contact chamber can be continued and thetreatment of the gas can be continued, thus providing a furtherimprovement in safety.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram showing an embodiment of apolyisocyanate production system of the present invention.

FIG. 2 is a schematic block diagram showing a detoxification column as afirst embodiment of the gas treatment apparatus of the presentinvention.

FIG. 3 is a schematic block diagram showing a detoxification column as asecond embodiment of the gas treatment apparatus of the presentinvention, and

FIG. 4 is a schematic block diagram showing a conventional type ofdetoxification column disclosed in a known literature.

EXPLANATION OF LETTERS OR NUMERALS

-   -   1: Polyisocyanate production system    -   3: Isocyanate producing reactor    -   4: Hydrogen chloride purifying tank    -   5: Hydrogen chloride absorbing column    -   6: Hydrogen chloride oxidation reactor    -   7: Detoxification column    -   21: Concentration control unit    -   22: Pressure control valve    -   23: Flow-rate control valve    -   24: Valve    -   25: Pressure sensor    -   32: Hydrochloric gas control unit    -   51: Detoxification column    -   55: Gas-liquid contact chamber    -   56: Storage chamber    -   57: Treatment liquid supply pipe    -   61: Harmful gas charge pipe    -   62: Treated gas discharge pipe    -   63: Back-flow pipe    -   64: Transport pipe

EMBODIMENTS OF THE INVENTION

FIG. 1 is a schematic block diagram showing an embodiment of apolyisocyanate production system of the present invention.

As shown in FIG. 1, a polyisocyanate production system 1 comprises acarbonyl chloride producing reactor 2, an isocyanate producing reactor 3serving as a polyisocyanate producing means, a hydrogen chloridepurifying tank 4 serving as a hydrogen chloride purifying means, ahydrogen chloride absorbing column 5 serving as a first detoxifyingtreatment means and a hydrochloric acid producing means, a hydrogenchloride oxidation reactor 6 serving as a chlorine producing means, anda detoxification column 7 serving as a second detoxifying treatmentmeans.

The carbonyl chloride producing reactor 2 is not limited to anyparticular one as long as it is a reactor for a reaction of chlorine(Cl₂) with carbon monoxide (CO) to produce carbonyl chloride (COCl₂).For example, the carbonyl chloride producing reactor 2 includes a fixedbed reactor in which an activated carbon catalyst is packed. Thecarbonyl chloride producing reactor 2 is connected to an isocyanateproducing reactor 3 via a connection line 8.

Chlorine gas and carbon monoxide gas, which are raw materials ofcarbonyl chloride, are supplied to the carbonyl chloride producingreactor 2 in such a proportion that a ratio (molar ratio) of carbonmonoxide to chlorine is 1.01/1-10/1. When chlorine is oversupplied,there is a possibility that an aromatic ring of polyisocyanate and ahydrocarbon group may be chlorinated in the isocyanate producing reactor3 by the oversupplied chlorine.

An amount of chlorine gas supplied and an amount of carbon monoxidesupplied are properly determined on the basis of an amount ofpolyisocyanate produced and an amount of hydrogen chloride gas producedsecondarily.

In the carbonyl chloride producing reactor 2, chlorine and carbonmonoxide are reacted with each other to produce carbonyl chloride. Inthis reaction, the carbonyl chloride producing reactor 2 is set at0-250° C. and 0-5 MPa-guage, for example.

The carbonyl chloride obtained may be cooled and liquefied properly tobe in a liquefied state in the carbonyl chloride producing reactor 2 ormay be absorbed properly in an adequate solvent to be a solution. Thecarbonyl chloride obtained is re-supplied to the carbonyl chlorideproducing reactor 2 according to need, after the carbon monoxide thereinis removed.

When at least a part of carbonyl chloride is in a liquefied state and/orsolution state, a concentration of carbon monoxide contained in thecarbonyl chloride can be reduced. This can provide an improvedconversion ratio of hydrochloric gas to chlorine in an oxidationreaction of hydrogen chloride mentioned later.

For producing liquefied carbonyl chloride, a condenser is provided inthe carbonyl chloride producing reactor 2 at a portion thereof on thedownstream side of the fixed bed reactor mentioned above, so that thecarbonyl chloride obtained can be liquefied by that condenser. For thisliquefying process, it is preferable that a concentration of carbonmonoxide contained in the carbonyl chloride is 1% by weight or less.

Then, the carbonyl chloride thus obtained is supplied to the isocyanateproducing reactor 3 via the connection line 8. The isocyanate producingreactor 3 is not limited to any particular one, as long as it is areaction tank for a reaction of carbonyl chloride with polyamine toproduce polyisocyanate. The isocyanate producing reactor 3 includes areactor provided with a stirring vane and a reaction column having aperforated plate. Preferably, the isocyanate producing reactor 3 isconfigured as a multistage tank. An adequate solvent or gas inactive topolyisocyanate is used for the isocyanate reaction. The isocyanateproducing reactor 3 is connected to the hydrogen chloride purifying tank4 through the connection line 9.

As raw materials, carbonyl chloride and polyamine are supplied from thecarbonyl chloride producing reactor 2 to the isocyanate producingreactor 3 through the connection line 8.

The carbonyl chloride is supplied from the carbonyl chloride producingreactor 2 in the condition of a gaseous state or in the condition of aliquefied and/or solution state in such a proportion that a ratio (molarratio) of carbonyl chloride to polyamine is 2/1-60/1.

The polyamine used is a polyamine corresponding to a polyisocyanate usedin a production of polyurethane. No particular limitation is imposed onthe polyamine. For example, the polyamine is properly selected fromaromatic diamines, such as polymethylenepolyphenylene polyamine (MDA)corresponding to polymethylenepolyphenylene polyisocyanate (MDI) andtolylenediamine (TDA) corresponding to tolylene diisocyanate (TDI),aralkyl diamines, such as xylylenediamine (XDA) corresponding toxylylenediisocyanate (XDI) and tetramethylxylylene diamine (TMXDA)corresponding to tetramethylxylylene diisocyanate (TMXDI), alicyclicdiamines, such as bis(aminomethyl) norbornane (NBDA) corresponding tobis(isocyanatomethyl) norbornane (NBDI),3-aminomethyl-3,5,5-trimethylcyclohexyl amine (IPDA) corresponding to3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate (IPDI),4,4′-methylenebis(cyclohexylamine) (H₁₂MDA) corresponding to4,4′-methylenebis(cyclohexylisocyanate) (H₁₂MDI), and bis(aminomethyl)cyclohexane (H₆XDA) corresponding to bis(isocyanatomethyl)cyclohexane(H₆XDI), aliphatic diamines, such as hexamethylene diamine (HDA)corresponding to hexamethylene diisocyanate (HDI), andpolymethylenepolyphenyl polyamine corresponding topolymethylenepolyphenyl polyisocyanate (crude MDI, polymeric MDI).

The polyisocyanate production system 1 is suitable for producingaromatic diisocyanate and polymethylenepolyphenyl polyisocyanate fromaromatic diamine and polymethylene polyphenyl polyamine.

Although the polyamine may be supplied directly, it is preferable thatthe polyamine is dissolved in a solvent to be a solution of 5 to 50% byweight.

The solvent used is not limited to any particular one. The solvents thatmay be used include the following: aromatic hydrocarbons such as tolueneand xylene; halogenated hydrocarbons such as chlorotoluene,chlorobenzene and dichlorobenzene; esters such as butyl acetate and amylacetate, and ketones such as methylisobutyl ketone and methylethylketone. Preferably, chlorobenzene and dichlorobenzene can be cited.

In the isocyanate producing reactor 3, the carbonyl chloride and thepolyamine undergo an isocyanate reaction to produce polyisocyanate,while hydrochloric gas (HCl gas) is also produced secondarily. In theisocyanate reaction, the above-mentioned solvent is added in theisocyanate producing reactor 3 separately or together with polyamine inthe isocyanate producing reactor 3 and the isocyanate producing reactor3 is set at 0-250° C. and 0-5 MPa-guage, for example.

The polyisocyanate undergoes an aftertreatment such as degasification,desolvating, and tar cutting, then is purified and provided as a rawmaterial of polyurethane.

The hydrochloric gas produced secondarily is supplied to the hydrogenchloride purifying tank 4 via the connection line 7, together with anentrained solvent and carbonyl chloride

The hydrogen chloride purifying tank 4 is not limited to any particularone as long as it can purify the hydrochloric gas by separating theentrained solvent and carbonyl chloride therefrom. For example, thehydrogen chloride purifying tank 4 includes a tray column and a packedcolumn which are provided with a condenser.

The hydrogen chloride purifying tank 4 is connected to the hydrogenchloride absorbing column 5 through a first hydrochloric gas connectionline 10. Also, the hydrogen chloride purifying tank 4 is connected tothe hydrogen chloride oxidation reactor 6 through a second hydrochloricgas connection line 11. Further, the hydrogen chloride purifying tank 4is connected to the detoxification column 7 through a third hydrochloricgas connection line 12.

A pressure control valve 22 serving as a first adjusting means foradjusting an amount of hydrochloric gas supplied is interposed in thefirst hydrochloric gas connection line 10 via which the hydrochloric gasis supplied from the hydrogen chloride purifying tank 4 to the hydrogenchloride absorbing column 5. Also, a flow-rate control valve 23 servingas a second adjusting means of the first opening and closing means foradjusting an amount of hydrochloric gas supplied is interposed in thesecond hydrochloric gas connection line 11 via which the hydrochloricgas is supplied from the hydrogen chloride purifying tank 4 to thehydrogen chloride absorbing column 5. Also, a valve 24 serving as asecond opening and closing means is interposed in the third hydrochloricgas connection line 12 via which the hydrochloric gas is supplied fromthe hydrogen chloride purifying tank 4 to the detoxification column 7. Aflowmeter 33 is interposed in the third hydrochloric gas connection line12 at a location on the upstream side of the valve 24. A pressure sensor25 serving as an abnormality detecting means for detecting the innerpressure of the tower is provided in the hydrogen chloride purifyingtank 4, so that the inner pressure of the tank is kept at e.g. 0.05-0.6MPa.

The pressure control valve 22, the flow-rate-control valve 23, the valve24, the flowmeter 33, and the pressure sensor 25 are connected to ahydrochloric gas control unit 32 serving as a control means. Aconnection switching means is composed by the flow-rate-control valve23, the valve 24, and the hydrochloric gas control unit 32. The innerpressure of the hydrochloric gas purifying tank 4 detected by thepressure sensor 25 is input to the hydrochloric gas control unit 32.

When the inner pressure of the hydrochloric gas purifying tank 4detected by the pressure sensor 25 is not more than a predeterminedlevel (e.g. 0.6 MPa), the hydrochloric gas control unit 32 regards thehydrogen chloride oxidation reactor 6 as normal and controls theflow-rate control valve 23 to connect the hydrogen chloride oxidationreactor 6 to the hydrogen chloride purifying tank 4 and also controlsthe valve 24 to disconnect the detoxification column 7 from the hydrogenchloride purifying tank 4. On the other hand, due to an abnormality ofthe hydrogen chloride oxidation reactor 6, the flow-rate control valve23 is shutoff or an opening of the flow-rate control valve 23 is reducedrapidly, to reduce a flow-rate of the flowmeter 33 rapidly. In thiscase, when an amount of hydrochloric gas produced secondarily from theisocyanate producing reactor 3 exceeds the treating capability of thehydrogen chloride absorbing column 5 so that the inner pressure of thehydrochloric gas purifying tank 4 exceeds a predetermined level (e.g.0.6 MPa) accordingly, the hydrochloric gas control unit 32 controls thevalve 24 to connect the detoxification column 7 to the hydrogen chloridepurifying tank 4.

In the hydrogen chloride purifying tank 4, the carbonyl chloride iscondensed by the condenser or absorbed by the solvent, so as to beseparated from the hydrochloric gas, and also a little amount of asolvent in the hydrochloric gas is separated from the hydrochloric gasby being absorbed in an activated carbon and the like.

In the hydrogen chloride purifying tank 4, a concentration of an organicmaterial in the hydrochloric gas is preferably reduced to be 1% byweight or less, or preferably 100 ppm or less, and a concentration ofcarbon monoxide in the hydrochloric gas is preferably reduced to be 10%(V/V) (percent by volume) or less, or preferably 3% (V/V) or less. Byreducing impurities in the hydrochloric gas to such a low level,disadvantageous effects on the catalyst, such as a decreased activity ora partial deactivation of the catalyst, can be reduced or prevented inthe hydrogen chloride oxidation reaction mentioned later. This canachieve an improved basic unit, an improved oxidation reaction of thehydrogen chloride, and an equalized temperature distribution of thehydrogen chloride oxidation reactor 6 to stabilize the hydrogen chlorideoxidation reactor 6. Further, this can improve a conversion ratio ofhydrochloric gas to chlorine.

Since the hydrogen chloride oxidation reactor 6 and the hydrogenchloride absorbing column 5 are connected in parallel to the hydrogenchloride purifying tank 4, when the hydrogen chloride oxidation reactor6 is in normal, the most of the purified hydrochloric gas is suppliedthe hydrogen chloride oxidation reactor 6 via the second hydrochloricgas connection line 11 and a surplus of the purified hydrochloric gas isdischarged to the hydrogen chloride absorbing column 5. A proportionbetween the hydrochloric gas supplied to the hydrogen chloride oxidationreactor 6 and the hydrochloric gas discharged to the hydrogen chlorideabsorbing column 5 is properly determined based on an ability of thehydrogen chloride oxidation reactor 6 to produce chlorine and an abilityof hydrogen chloride absorbing column 5 to produce hydrochloric acid.

The hydrogen chloride oxidation reactor 6 is not limited to anyparticular one as long as it is a reactor tank for oxidizing thehydrochloric gas to produce chlorine (Cl₂). For example, the hydrogenchloride oxidation reactor 6 includes a fluid bed reactor using chromiumoxide as a catalyst and a fixed bed reactor using ruthenium oxide as thecatalyst. The hydrogen chloride oxidation reactor 6 is connected to thecarbonyl chloride producing reactor 2 via a re-supply line 13 andconnected to the hydrogen chloride absorbing tank 5 via a hydrochloricacid connection line 14.

When the hydrogen chloride oxidation reactor 6 is comprising the fluidbed reactor, at least 0.25 mol of oxygen per mol of hydrogen chloridecontained in the hydrochloric gas is supplied to the fluid bed reactor,for a reaction in the presence of chromium oxide at 0.1-5 MPa-gauge and300-500° C. with reference to Japanese Laid-open (Unexamined) PatentPublication No. Sho 62-275001, for example. An amount of hydrochloricgas supplied is in a range of e.g. 0.2-1.8 Nm³/h·kg-catalyst.

When the hydrogen chloride oxidation reactor 6 is comprising the fixedbed reactor, at least 0.25 mol of oxygen per mol of hydrogen chloridecontained in the hydrochloric gas is supplied to the fixed bed reactor,for a reaction in the presence of a ruthenium-containing catalyst at0.1-5 MPa and 200-500° C. with reference to Japanese Laid-open(Unexamined) Patent Publication No. 2000-272906, for example.

Then, in the hydrogen chloride oxidation reactor 6, the hydrochloric gasis oxidized by oxygen (O₂), whereby chlorine is produced and water (H₂O)is produced secondarily. As a result of this, chlorine and hydrochloricacid (an aqueous solution of hydrogen chloride: HCl/H₂O) are produced.In this oxidation reaction, a conversion ratio of hydrochloric gas tochlorine is e.g. 60% or more, or preferably 70-95%.

Then, in the polyisocyanate production system 1, the chlorine obtainedin the hydrogen chloride oxidation reactor 6 is supplied to the carbonylchloride producing reactor 2 via the re-supply line 13 and is reused asa raw material for producing carbonyl chloride in the carbonyl chlorideproducing reactor 2. When the chlorine thus obtained is reused as theraw material of the carbonyl chloride, the chlorine can be recycledwithout being discharged to the outside of the system of thepolyisocyanate production system 1. This enables an efficient use of thehydrochloric gas produced secondarily, while reducing environmentalloads.

In this polyisocyanate production system 1, chlorine (additionalchlorine) prepared separately as a raw material is also supplied to thecarbonyl chloride producing reactor 2 according to need, other than thechlorine (the recycled chlorine) supplied thereto from the hydrogenchloride oxidation reactor 6 via the re-supply line 13. The additionalchlorine may be purchased from outside. Alternatively, a separatechlorine production apparatus using a process such as electrolyzationmay be provided so as to supply the additional chlorine therefrom.

Although unoxidized (unreacted) hydrochloric gas and hydrochloric acidproduced secondarily in the hydrogen chloride oxidation reactor 6 may beprovided to a production of hydrochloric acid of a predeterminedconcentration in the hydrogen chloride oxidation reactor 6 forindustrial use or other process as an acid catalyst ofpolymethylenepolyphenylene polyamine (MDA), the unoxidized (unreacted)hydrochloric gas and the hydrochloric acid produced secondarily in thehydrogen chloride oxidation reactor 6 are supplied to the hydrogenchloride absorbing column 5 via the hydrochloric acid connection line14, for example.

That is, the hydrochloric gas is converted to chlorine at a constantconversion ratio in the hydrogen chloride oxidation reactor 6.Accordingly, the unoxidized (unreacted) hydrochloric gas and thehydrochloric acid produced secondarily corresponding to the rest fromhydrochloric gas converted to the chlorine are supplied from thehydrogen chloride oxidation reactor 6 to the hydrogen chloride absorbingcolumn 5 via the hydrochloric acid connection line 14 at a fixed ratio.For example, where a conversion ratio in the hydrogen chloride oxidationreactor 6 is 80%, 80% of the hydrochloric gas is converted to chlorinein the hydrogen chloride oxidation reactor 6, while on the other hand,the unoxidized (unreacted) hydrochloric gas and the hydrochloric acidproduced secondarily corresponding to the remaining 20% of thehydrochloric gas are supplied therefrom to the hydrogen chlorideabsorbing column 5 via the hydrochloric acid connection line 14.

The hydrogen chloride absorbing column 5 is not limited to anyparticular one as long as it can adjust a hydrochloric acid solution (asolution of hydrogen chloride: HClaq) by absorbing hydrochloric gas inwater, and comprised of a known absorbing column.

In the hydrogen chloride absorbing column 5, hydrochloric gas dischargedfrom the hydrogen chloride purifying tank 4 via the firsthydrochloric-gas connection line 10 and the unoxidized (unreacted)hydrochloric gas supplied from the hydrogen chloride oxidation reactor 6via the hydrochloric acid connection line 14 are absorbed in water tothereby produce hydrochloric acid. In addition to these, hydrochloricacid produced secondarily supplied from the hydrogen chloride oxidationreactor 6 via the hydrochloric acid connection line 14 is also addedthereto. The hydrochloric acid obtained, as it is, or purified by anactivated carbon and the like, is provided for industrial use.

When the unoxidized hydrochloric gas in the hydrogen chloride oxidationreactor 6 and the hydrochloric acid produced therein are supplied to thehydrogen chloride absorbing column 5 without being discharged in themanner described above, hydrochloric acid can be produced effectively,while utilizing an efficient use of surplus hydrochloric gas. When thehydrochloric acid is produced from the surplus hydrochloric gas in thehydrogen chloride oxidation reactor 6, the surplus hydrochloric gas canbe detoxified while the hydrochloric acid produced therefrom can bereused. As a result of this, an effective use of the surplushydrochloric gas can be realized.

A water supply line 15 for supplying water for the hydrochloric gas tobe absorbed therein, a hydrochloric acid discharge line 16 fordischarging the hydrochloric acid obtained, and a hydrochloric acidback-flow line 17 one end of which is connected to the hydrochloric aciddischarge line 16 and other end of which is connected to the hydrogenchloride absorbing column 5 are connected to the hydrogen chlorideabsorbing column 5. A water supply regulating valve 18 is interposed inthe water supply line 15, and a back-flow regulating valve 19 isinterposed in the hydrochloric acid back-flow line 17. A concentrationsensor 20 is interposed in the hydrochloric acid discharge line 16. Thewater supply regulating valve 18, the back-flow regulating valve 19, andconcentration sensor 20 are connected to a concentration control unit21, which serve as hydrochloric acid concentration adjusting means.

In the hydrogen chloride absorbing column 5, water is supplied to fromthe water supply line 15, and the hydrochloric gas discharged from thehydrochloric acid purifying tank 4 via the first hydrochloric gasconnection line 10 and the unoxidized (unreacted) hydrochloric gassupplied thereto from the hydrogen chloride oxidation reactor 6 via thehydrochloric acid connection line 14 are absorbed in the water.Thereafter, the hydrochloric acid thus obtained is discharged from thehydrochloric acid discharge line 16. Further, a part of the hydrochloricacid flows back to the hydrogen chloride absorbing column 5 withoutbeing discharged from the hydrochloric acid discharge line 16.

In the hydrogen chloride absorbing column 5, a concentration of thehydrochloride acid discharged from the hydrochloric acid discharge line16 is monitored by the concentration sensor 20. The concentration of thehydrochloride acid is input to the concentration control unit. Theconcentration control unit 21 controls the water supply regulating valve18 and the back-flow regulating valve 19 based on the inputconcentration of the hydrochloric acid to adjust an amount of watersupplied from the water supply line 15 and an amount of hydrochloricacid flown back from the hydrochloric acid discharge line 16, wherebythe concentration of the hydrochloric acid discharged from thehydrochloric acid discharge line 16 is adjusted to a desiredconcentration.

When the concentration of the hydrochloric acid discharged from thehydrochloric acid discharge line 16 is adjusted to a desiredconcentration as is described, the hydrochloric acid of stable qualitycan be produced. The hydrochloric acid of a concentration thus adjustedto e.g. 30-37 weight % can be used as it is for industrial use.

The detoxification column 7 includes a treatment tank 26, a storage tank27, and a pump 28. The treatment tank 26 has a gas-liquid contactchamber 29 in which a packed material is packed to improve an efficiencyof gas-liquid contact and also has showers 30 arranged over thegas-liquid contact chamber 29. A bottom of the treatment tank 26, thestorage tank 27, the pump 28, and the showers 30 are connected via acirculation line 31.

A sodium hydroxide aqueous solution (NaOHaq.) for detoxifying thehydrochloric gas is stored in the storage tank 27. The sodium hydroxideaqueous solution is first pumped up by the pump 28 flowing upwardthrough the circulation line 31, and sprayed from the showers 30 intothe gas-liquid contact chamber 29 of the treatment tank 26. Afterpassing through the gas-liquid contact chamber 29, the sodium hydroxideaqueous solution flows back to the storage tank 27 from the bottom ofthe treatment tank 26. The sodium hydroxide aqueous solution iscirculated in the sequence as described. A part of the sodium hydroxideaqueous solution is discharged from the circulation line 31 to adjust aconcentration of the sodium hydroxide aqueous solution in the storagetank 27 to a predetermined concentration of e.g. 5-30%.

On the other hand, the third hydrochloric gas connection line 12 isconnected to the treatment tank 26 so as to flow upward from the bottomof the gas-liquid contact chamber 29. The detoxification column 7 andthe hydrogen chloride absorbing column 5 are connected in parallel tothe hydrogen chloride purifying tank 4. When an abnormality is in thehydrogen chloride oxidation reactor 6, the hydrochloric gas dischargedfrom the third hydrochloric gas connection line 12 is supplied to thedetoxification column 7 so as to contact with the sodium hydroxideaqueous solution sprayed from the showers 30 in the gas-liquid contactchamber 29 in the vertically opposite direction for an effectivegas-liquid contact to detoxify the hydrochloric gas. Thereafter, theresultant gas is discharged from the treatment tank 26 to theatmosphere.

As described above, in this polyisocyanate production system 1, when anabnormality of the hydrogen chloride oxidation reactor 6 is not detectedby the pressure sensor 25, the hydrochloric gas purified by the hydrogenchloride purifying tank 4 is supplied to the hydrogen chloride oxidationreactor 6. Then, chlorine is produced from the hydrochloric gas suppliedin the hydrogen chloride oxidation reactor 6 and discharged to thehydrogen chloride absorbing column 5. Then, the hydrochloric acid isproduced from the hydrochloric gas discharged in the hydrogen chlorideabsorbing column 5. The hydrochloric gas is detoxified in the processesdescribed above.

On the other hand, when an abnormality is in the hydrogen chlorideoxidation reactor 6 and then an amount of the hydrogen chloride exceedsthe treating capability of the hydrogen chloride absorbing column 5, sothat the abnormality of the hydrogen chloride oxidation reactor 6 isdetected by the pressure sensor 25, the hydrochloric gas control unit 32controls the valve 24 to discharge the hydrogen chloride from thehydrogen chloride purifying tank 4 to the detoxification column 7, so asto adjust the pressure of the hydrogen chloride purifying tank 4 to apredetermined pressure. As a result of this, the hydrochloric gassupplied to the hydrogen chloride oxidation reactor 6 in the interim isdischarged to the detoxification column 7 and is detoxified in thedetoxification column 7. As a result, when the hydrogen chlorideoxidation reactor 6 is in normal, the hydrochloric gas can be suppliedsteadily to the hydrogen chloride oxidation reactor 6, while a surplusof the hydrochloric gas is detoxified in the hydrogen chloride absorbingcolumn 5. When a trouble occurs in the hydrogen chloride oxidationreactor 6, the hydrochloric gas supplied to the hydrogen chlorideoxidation reactor 6 in the interim is detoxified in the detoxificationcolumn 7. Thus, a large amount of hydrochloric gas supplied to thehydrogen chloride oxidation reactor 6 in the interim can be detoxifiedaccording to the ability of the hydrogen chloride absorbing column 5 toproduce the hydrochloric acid, thus achieving an effective treatment ofthe hydrochloric gas.

When an abnormality of the hydrogen chloride oxidation reactor 6, suchas a temperature anomaly of the hydrogen chloride oxidation reactor 6,is detected, the hydrogen chloride oxidation reactor 6 is operated foran interlock control to stop the production of chlorine. Also, thedetoxification column 7 is set to detoxify the hydrochloric gas for apredetermined time (e.g. 30 minutes), during which the production ofpolyisocyanate in the isocyanate producing reactor 3 is shut down stablyto secure a safe shutdown.

In this polyisocyanate production system 1, when the hydrogen chlorideoxidation reactor 6 is in normal, the hydrochloric gas control unit 32controls the flow-rate control valve 23 to keep (e.g. 90 where thehydrochloric gas purified in the hydrogen chloride purifying tank 4 istaken to be 100) an amount of hydrochloric gas supplied from thehydrogen chloride purifying tank 4 to the hydrogen chloride oxidationreactor 6 via the second hydrochloric-gas connection line 11 to beconstant. At the same time, the hydrochloric gas control unit 32 alsocontrols the pressure control valve 22 based on the inner pressure ofthe hydrogen chloride purifying tank 4 input from the pressure sensor 25to discharge the hydrochloric gas from the hydrogen chloride purifyingtank 4 to the hydrogen chloride absorbing column 5 via the firsthydrochloric-gas connection line 10 so as to keep the inner pressure ofthe hydrogen chloride purifying tank 4 (where the hydrochloric gaspurified in the hydrogen chloride purifying tank 4 is taken to be 100,the remaining amount (10) of 90 supplied to the second hydrochloric gasconnection line 11 is discharged, for example).

When the hydrochloric gas control unit 32 controls the flow-rate controlvalve 23 and the pressure control valve 22 in the manner describedabove, the hydrochloric gas is steadily supplied to the hydrogenchloride oxidation reactor 6 at a constant flow-rate, while the surplusof the hydrochloric gas is discharged to the hydrogen chloride absorbingcolumn 5, whereby the inner pressure of the hydrogen chloride purifyingtank 4 and thus the inner pressure of the isocyanate producing reactor 3can be kept constant. As a result, this can produce the chlorinesteadily from the hydrochloric gas produced secondarily, while the innerpressure of the isocyanate producing reactor 3 can be kept constant.This enables a stable reaction between carbonyl chloride and polyamineand an effective treatment of the hydrochloric gas produced secondarily.

In this polyisocyanate production system 1, the concentration controlunit 21 and the hydrochloric gas control unit 32 are connected via a busand controlled in a central control unit. This builds a distributedcontrol system of the polyisocyanate production system 1.

FIG. 2 is a schematic block diagram showing a detoxification column 51Ato detoxify exhaust gases as a first embodiment of the gas treatmentapparatus of the present invention.

As shown in FIG. 2, the detoxification column 51A is provided in theplant for producing chemical products in order to detoxify a harmful gasas an exhaust gas produced in the chemical process and then dischargethe detoxified gas to atmosphere. The harmful gas includes, for example,a carbonyl-chloride-containing gas and a hydrogen-chloride-containinggas produced in the polyisocyanate production process in thepolyisocyanate production plant. Accordingly, the detoxification column51A can be used as the detoxification column 7 of the polyisocyanateproduction system 1.

The detoxification column 51A is comprising a treatment tank 52, astorage tank 53, and a solution sending pump 54 used as a power source.

The treatment tank 52 is in a closed cylindrical form extendingvertically and closed at its upper end and lower end, comprising agas-liquid contact chamber 55, a storage chamber 56, and a treatmentliquid supply pipe 57 serving as a treatment liquid supply means.

The treatment tank 52 is provided, at a lower portion thereof, with apacked material support plate 58 for vertically demarking an interiorspace of the treatment tank 52, and at an upper portion thereof, with astorage chamber bottom plate 59, spaced above from the packed materialsupport plate 58 for vertically demarking the interior space of thetreatment tank 52 into the gas-liquid contact chamber 55 and the storagechamber 56. The packed material support plate 58 has a number of air(liquid) vents bored therein.

The gas-liquid contact chamber 55 is defined as an interior space of thetreatment tank 52 between the packed material support plate 58 and thestorage chamber bottom plate 59. In the gas-liquid contact chamber 55,packed materials such as a Rasching ring, a Berl saddle and the like,are packed on the packed material support plate 58 to a level to leave aspace between the packed material and the storage chamber bottom plate59.

The storage chamber 56 is defined as an interior space of the treatmenttank 52 located above the storage chamber bottom plate 59.

The treatment liquid supply pipe 57, one end of which is connected to alower portion of the storage chamber 56 and the other end of which isinserted in the gas-liquid contact chamber 55, is located above thepacked material. A restriction orifice 60 for restricting a flow-rate ofthe treatment liquid flowing through the treatment liquid supply pipe 57is interposed in the treatment liquid supply pipe 57. At a lower portionof the treatment liquid pipe, a shower or a liquid disperser, though notshown, is provided to spray the treatment liquid uniformly over thepacked material.

A harmful gas charge pipe 61 for flowing in the harmful gas to thetreatment tank 52 is connected to the treatment tank 52 at a lower sideof the packed material support plate 58, or on the underside of thegas-liquid contact chamber 55. A treatment gas discharge pipe 62 forflowing out the harmful gas detoxified (hereinafter it is referred asthe treated gas) to from the treatment tank 52 is connected to thetreatment tank 52 at a lower side of the storage chamber bottom plate59, or above of the packed material in the gas-liquid contact chamber55.

The treatment liquid for detoxifying the harmful gas is stored in thestorage tank 53. When the harmful gas is, for example, thecarbonyl-chloride-containing gas or the hydrogen-chloride-containing gascited above, an alkaline aqueous solution, such as a sodium hydroxideaqueous solution or a potassium hydroxide aqueous solution, is used asthe treatment liquid. The treatment liquid is properly re-supplied tothe storage tank 53 according to a circulating amount of the treatmentliquid.

A back-flow pipe 63, one end of which is connected to a lower end of thetreatment tank 52, is connected to the storage tank 53 at the other endthereof. The storage tank 53 is connected to the treatment tank 52 viathe back-flow pipe 63. A transport pipe 64, one end of which isconnected to the storage chamber 56 located above the treatment tank 52,is connected to the storage tank 53 at the other end thereof. Thestorage tank 53 is connected to the storage chamber 56 via the transportpipe 64.

The solution sending pump 54 is not limited to any particular one aslong as it can send a liquid, including a reciprocating pump, acentrifugal pump, and a rotary pump. The solution sending pump 54 isinterposed in the transport pipe 64.

Now, reference is given to a continuous steady operation of thedetoxification column 51A.

In the detoxification column 51A, when the solution sending pump 54 isdriven, the treatment liquid is pumped up from the storage tank 53 andsent to the storage chamber 56 through the transport pipe 64.

The treatment liquid sent to the interior of the storage chamber 56flows in the treatment liquid supply pipe 57. After a flow-rate of thetreatment liquid is restricted by the restriction orifice 60, thetreatment liquid discharged from the treatment liquid supply pipe 57with a gravity-drop and is sprayed over the packed material from abovein the interior of the gas-liquid contact chamber 55. At this time, anoverflow line may be provided in the storage chamber 56 so that anoverflowing treatment liquid can also be sprayed in the gas-liquidcontact chamber 55.

On the other hand, the harmful gas flows into the treatment tank 52 fromthe harmful gas charge pipe 61 and then flows into the gas-liquidcontact chamber 55 via the air vents of the packed material supportplate 58 passing through spaces between the packed materials from bottomto top. As a result of this, the harmful gas contacts with the treatmentliquid sprayed from above in the vertically opposite direction for aneffective gas-liquid contact to detoxify the harmful gas. For example,in the case that the harmful gas is the carbonyl-chloride-containing gasand the treatment liquid is a sodium hydroxide aqueous solution, whenthese undergo the gas-liquid contact, sodium chloride and sodiumcarbonate are produced and the harmful gas is detoxified. Thereafter,the treated gas is discharged to the atmosphere via the treated gasdischarge pipe 62.

Thereafter, the treatment liquid dropping from the interior of thegas-liquid contact chamber 55 via the air vents of the packed materialsupport plate 58 returns to the storage tank 53 from the lower end ofthe treatment tank 52 via the back-flow pipe 63. As seen from above, inthe detoxification column 51A, a circulation line 67 serving as atreatment liquid back-flow means is composed by the transport pipe 64and the back-flow pipe 63. The treatment liquid is circulated throughthe circulation line 67 by the drive of the solution sending pump 54.

When the treatment liquid is circulated in the manner described above,waste of the treatment liquid can be minimized to reduce running costs.

In this detoxification column 51A, the continuous steady operation iscontinued usually. In this detoxification column 51A, even when thedrive of the solution sending pump 54 is stopped due to a trouble, suchas an electric power failure and a malfunction so as that thecirculation of the treatment liquid through the circulation line 67 isstopped, since the storage chamber 56 is located over the gas-liquidcontact chamber 55, the treatment liquid stored in the storage chamber56 drops by gravity into the gas-liquid contact chamber 55 through thetreatment liquid supply pipe 57 without requiring any specific powersource. Then, the treatment liquid is supplied to the interior of thegas-liquid contact chamber 55 via the treatment liquid supply pipe 57until the treatment liquid in the storage chamber 56 runs out. Thus,even when the supply of the treatment liquid to the gas-liquid contactchamber 55 is interrupted due to a trouble resulted from the solutionsending pump 54, the gas-liquid contact between the treatment liquid andthe harmful gas in the gas-liquid contact chamber 55 can be continued tocontinue the treatment of the harmful gas, thus providing a furtherimprovement in safety.

Even when the treatment liquid in the storage chamber 56 runs out, sinceresidual treatment liquid remains in the gas-liquid contact chamber 55,the harmful gas can be detoxified until such residual treatment liquidis consumed.

When a power failure or a malfunction occurs during the time the harmfulgas is detoxified by the treatment liquid stored in the storage chamber56, the treatment liquid may be circulated again by switching the powerto an auxiliary power in the power failure or by driving to anotherliquid sending pump in the malfunction.

The flow-rate of the harmful gas flowing into the treatment tank 52 fromthe harmful gas charge pipe 61 and the flow-rate of the treatment liquidcirculated by the solution sending pump 54 are properly determineddepending on the types of the harmful gas or the treatment liquid, andthe types of the treatment for detoxification.

Also, an amount of treatment liquid stored in the storage chamber 56 isproperly determined depending on the types of the harmful gas or thetreatment liquid, and the treatment quantity of the plant.

FIG. 3 is a schematic block diagram showing a detoxification column 51Bas a second embodiment of the gas treatment apparatus of the presentinvention. In FIG. 3, the same numerals refer to corresponding membersto those shown in FIG. 2 and the description thereon is omitted.

The detoxification column 51B is configured as a double-stage continuousdetoxification column, comprising a first-stage detoxification column 51a and a second-stage detoxification column 51 b which are connected inseries in a flowing direction of the harmful gas.

The first-stage detoxification column 51 a is comprising a treatmenttank 52, a storage tank 53, and a solution sending pump 54, as is thecase with the detoxification column 51 of the first embodiment.

However, the treatment tank 52 of the first-stage detoxification column51 a is not provided with a treatment liquid supply pipe 57. Instead, anend of a transport pipe 64 is inserted in a gas-liquid contact chamber55 and is located above a packed material (the first-stagedetoxification column 51 a may be provided with the treatment liquidsupply pipe 57, if needed). A shower or a liquid disperser, though notshown, is provided at the end of the transport pipe 64 to spray thetreatment liquid uniformly over the packed material.

The second-stage detoxification column 51B is also provided with atreatment tank 52, a storage tank 53, and a solution sending pump 54, asis the case with the first embodiment. The second-stage detoxificationcolumn 51 b has the same configuration as that of the detoxificationcolumn 51 of the first embodiment.

In the detoxification column 51B, a treated gas discharge pipe 62 of thefirst-stage detoxification column 51 a serves a harmful gas charge pipe61 of the second-stage detoxification column 51 b as a gas passage pathis connected to the second-stage harm removing tank 51 b at a lower sidethe gas-liquid contact chamber 55 of the treatment tank 52 of thesecond-stage detoxification column 51 b.

In this detoxification column 51B, a storage chamber 56 of thefirst-stage detoxification column 51 a and a storage chamber 56 of thesecond-stage detoxification column 51 b are connected with each othervia a storage chamber communication pipe 65. The storage chambercommunication pipe 65 is connected at one end thereof to the storagechamber 56 of the first-stage detoxification column 51 a and isconnected at the other end to the treatment liquid supply pipe 57 of thesecond-stage detoxification column 51 at a mid-location thereof(upstream of the restrict orifice 60). The storage chamber 56 of thefirst-stage detoxification column 51 a and the storage chamber 56 of thesecond-stage detoxification column 51 b communicate with each other viathe storage chamber communication pipe 65. The storage chambercommunication pipe 65 is used for detoxification in the second-stagedetoxification column 51 b.

Also, in the detoxification column 51B, the storage tank 53 of thefirst-stage detoxification column 51 a and the storage tank 53 of thesecond-stage detoxification column 51 b are connected with each othervia a storage tank communication pipe 66. The storage tank communicationpipe 66 is connected at one end thereof to the storage tank 53 of thefirst-stage detoxification column 51 a and is connected at the other endto the storage tank 53 of the second-stage detoxification column 51 b.The storage tank 53 of the first-stage detoxification column 51 a andthe storage tank 53 of the second-stage detoxification column 51 bcommunicate with each other via the storage tank communication pipe 66.As a result of this, the treatment liquids of the both storage tanks 53are increased or decreased equally so that liquid levels (water levels)of the both storage tanks 53 correspond to each other.

Now, reference is given to a continuous steady operation of thedetoxification column 51B.

In the first-stage detoxification column 51 a of this detoxificationcolumn 51B, when the solution sending pump 54 is driven, the treatmentliquid is pumped up from the storage tank 53 and is sprayed over thepacked material from above of the packed material in the gas-liquidcontact chamber 55 via the transport pipe 64.

The treatment liquid thus sprayed contacts with the harmful gas whichflows in from the harmful gas charge pipe 61 and passes through thespaces between the packed materials packed in the gas-liquid contactchamber 55 from bottom toward top in the vertically opposite directionfor an effective gas-liquid contact, thereby detoxifying the harmfulgas. In the detoxification column 51B, a detoxification rate of theharmful gas in the first-stage detoxification column 51 b is set to beless than 100%, then the harmful gas is detoxified completely bydetoxifying the remaining harmful gas in the next second-stagedetoxification column 51 b.

Thereafter, the harmful gas detoxified in the first-stage detoxificationcolumn 51 a, or the treated gas is flown into the treatment tank 52 ofthe second-stage detoxification column 51 b through the treated gasdischarge pipe 62 of the first-stage detoxification column 51 a (theharmful gas charge pipe 61 of the second-stage detoxification column 51b).

The treatment liquid is returned to the storage tank 53 from the lowerend of the treatment tank 52 through the back-flow pipe 63 and iscirculated through the circulation line 67 formed by the transport pipe64 and the back-flow pipe 63.

In the second-stage detoxification column 51 b, when the solutionsending pump 54 is driven, the treatment liquid is pumped up from thestorage tank 53 and transported to the storage chamber 56 through thetransport pipe 64.

The treatment liquid sent to the interior of the storage chamber 56flows in the treatment liquid supply pipe 57. After a flow-rate of thetreatment liquid is restricted by the restriction orifice 60, thetreatment liquid is discharged from the treatment liquid supply pipe 57with a gravity-drop and is sprayed over the packed material from aboveof the packed material in the gas-liquid contact chamber 55. At thistime, an overflow line may be provided in the storage chamber 56 so thatan overflowing treatment liquid can also be sprayed in the gas-liquidcontact chamber 55.

The treatment liquid thus sprayed contacts with the harmful gas whichflows in from the treated gas discharge pipe 62 of the first-stagedetoxification column 51 a (the harmful gas charge pipe 61 of thesecond-stage detoxification column 51 b) and passes through the spacesbetween the packed materials packed in the gas-liquid contact chamber 55from bottom toward top in the vertically opposite direction for aneffective gas-liquid contact, thereby detoxifying the harmful gascompletely.

Thereafter, the harmful gas detoxified in the second-stagedetoxification column 51 b, or the treated gas is discharged toatmosphere via the treated gas discharge pipe 62.

The treatment liquid is returned to the storage tank 53 from the lowerend of the treatment tank 52 through the back-flow pipe 63 and iscirculated through the circulation line 67 formed by the transport pipe64 and the back-flow pipe 63.

In the detoxification column 51B as well, even when the drive of theboth solution sending pumps 54 are stopped due to a trouble, such as anelectric power failure and a malfunction so as that the treatment liquidis not circulated through the circulation lines 67, the treatment liquidstored in the storage chamber 56 drops by gravity into the gas-liquidcontact chamber 55 in the second-stage detoxification column 51 bimmediately before the treated gas is discharged to atmosphere (i.e.,one in the detoxification column of the multistage continuousdetoxification column located at a most downstream side with respect toa flowing direction of the harmful gas) through the treatment liquidsupply pipe 57 without requiring any specific power source, so that thegas-liquid contact between the treatment liquid and the harmful gas iscontinued. Thus, as is the case with the detoxification column 51A ofthe first embodiment, even when the supply of the treatment liquid tothe gas-liquid contact chamber 55 is interrupted due to a troubleresulted from the solution sending pump 54, the gas-liquid contactbetween the treatment liquid and the harmful gas in the gas-liquidcontact chamber 55 can be continued to continue the treatment of theharmful gas, thus providing a further improvement in safety.

Also, in the detoxification column 51B, a detoxification rate of theharmful gas in the first-stage detoxification column 51 b is set to beless than 100%, then the harmful gas is detoxified completely bydetoxifying the remaining harmful gas in the next second-stagedetoxification column 51 b. Hence, the treatment of detoxifying theharmful can be carried out multistage-wise and continuously, thusachieving an effective treatment of the harmful gas.

In the detoxification column 51B of the second embodiment in which onlythe second-stage detoxification column 51 b is provided with thetreatment liquid supply pipe 57 for a gravity drop of the treatmentliquid, the first-stage detoxification column 51 a may also be providedwith a corresponding treatment liquid supply pipe 57 for a gravity dropof the treatment liquid. However, the illustrated embodiment whereinonly the second-stage detoxification column 51 b immediately before thetreated gas is discharged to atmosphere (i.e., one in the detoxificationcolumn of the multistage continuous detoxification column located at amost downstream side with respect to a flowing direction of the harmfulgas) is provided with the treatment liquid supply pipe 57 for a gravitydrop of the treatment liquid can provide an advantage of providing asimplified structure of the system, while proving a further improvedsafety in the treatment of the harmful gas.

INDUSTRIAL APPLICABILITY

The polyisocyanate production system of the present invention issuitably used as a production apparatus for producing polyisocyanateused as a raw material of polyurethane.

Also, the gas treatment apparatus of the present invention is suitablyused as the apparatus for treating gas such as, for example, detoxifyinga harmful gas produced in a chemical process in the chemical productproducing plant.

1. A polyisocyanate production system comprising: a polyisocyanateproducing unit for producing polyisocyanate by reacting carbonylchloride with polyamine, a hydrogen chloride purifying means unit towhich hydrogen chloride produced secondarily in the polyisocyanateproducing unit is supplied to purify the hydrogen chloride, a chlorineproducing unit to which the hydrogen chloride purified in the hydrogenchloride purifying unit is supplied and in which the hydrogen chlorideis oxidized to produce the chlorine, a hydrochloric acid producing unitto which the hydrogen chloride purified in the hydrogen chloridepurifying unit is supplied and in which the hydrogen chloride isabsorbed in water to produce a hydrochloric acid, a first adjusting unitfor adjusting an amount of hydrogen chloride supplied from the hydrogenchloride purifying unit to the hydrochloric acid producing unit, asecond adjusting unit for adjusting an amount of hydrogen chloridesupplied from the hydrogen chloride purifying unit to the chlorineproducing unit, and a control unit for controlling the second adjustingunit so that an amount of hydrogen chloride supplied from the hydrogenchloride purifying unit to the chlorine producing unit is kept constantand controlling the first adjusting unit so that an inner pressure ofthe hydrogen chloride purifying unit is kept constant.
 2. Thepolyisocyanate production system according to claim 1, whereinunoxidized hydrogen chloride and hydrochloric acid in the chlorineproducing unit are supplied to the hydrochloric acid producing unit. 3.The polyisocyanate production system according to claim 1, wherein thehydrochloric acid producing unit is provided with ahydrochloric-acid-concentration adjusting unit for adjusting aconcentration of the hydrochloric acid produced.
 4. A polyisocyanateproduction system comprising: a polyisocyanate producing unit forproducing polyisocyanate by reacting carbonyl chloride with polyamine, ahydrogen chloride purifying unit connected to the polyisocyanateproducing unit for purifying the hydrogen chloride produced secondarilyin the polyisocyanate producing unit, a chlorine producing unitconnected to the hydrogen chloride purifying unit for producing chlorineby oxidizing the hydrogen chloride purified in the hydrogen chloridepurifying unit, a first detoxifying treatment unit connected to thehydrogen chloride purifying unit in parallel with respect to thechlorine producing unit for detoxifying the hydrogen chloride dischargedfrom the hydrogen chloride purifying unit, a second detoxifyingtreatment unit selectively connected to the hydrogen chloride purifyingunit with respect to the chlorine producing means unit for detoxifyingthe hydrogen chloride discharged from the hydrogen chloride purifyingunit, an abnormality detecting unit for detecting an abnormality of thehydrogen chloride purifying unit, and a connection switching unit whichconnects the chlorine producing unit with the hydrogen chloridepurifying unit when an abnormality is not detected by the abnormalitydetecting unit and connects the second detoxifying treatment unit withthe hydrogen chloride purifying unit when an abnormality is detected bythe abnormality detecting unit.
 5. The polyisocyanate production systemaccording to claim 4, wherein the connection switching unit comprises: afirst opening and closing unit for connecting the chlorine producingunit to or disconnect from the hydrogen chloride purifying means unit, asecond opening and closing unit for connecting the second detoxifyingtreatment unit to or disconnecting from the hydrogen chloride purifyingunit, and a control unit for controlling the first opening and closingmeans unit and the second opening and closing unit, wherein when anabnormality is not detected by the abnormality detecting unit, thecontrol unit controls the first opening and closing unit to connect thechlorine producing unit with the hydrogen chloride purifying unit andalso controls the second opening and closing unit to disconnect thesecond detoxifying treatment unit from the hydrogen chloride purifyingunit, while on the other hand, when an abnormality is detected by theabnormality detecting unit, the control unit controls the first openingand closing unit to disconnect the chlorine producing unit from thehydrogen chloride purifying unit or to reduce the hydrogen chloridepurified in the hydrogen chloride purifying unit rapidly in an amountsupplied to the chlorine producing unit and also controls the secondopening and closing unit to connect the second detoxifying treatmentunit with the hydrogen chloride purifying unit according to a treatingcapability of the first detoxifying treatment unit.
 6. Thepolyisocyanate production system according to claim 4, wherein the firstdetoxifying treatment unit is a hydrochloric acid producing unit forproducing hydrochloric acid by absorbing the hydrogen chloride in water.7. The polyisocyanate production system according to claim 6, whereinthe unoxidized hydrogen chloride and hydrochloric acid in the chlorineproducing unit are supplied to the hydrochloric acid producing meansunit.
 8. The polyisocyanate production system according to claim 6,which comprises: a first adjusting unit for adjusting an amount ofhydrogen chloride supplied from the hydrogen chloride purifying unit tothe hydrochloric acid producing unit, a second adjusting unit foradjusting an amount of hydrogen chloride supplied from the hydrogenchloride purifying unit to the chlorine producing unit, and a controlunit for controlling the second adjusting unit so that an amount ofhydrogen chloride supplied from the hydrogen chloride purifying meansunit to the chlorine producing unit is kept constant and controlling thefirst adjusting unit so that an inner pressure of the hydrogen chloridepurifying unit is kept constant.
 9. A gas treatment apparatus fortreating a gas by bringing the gas into contact with a treatment liquid,the gas treatment apparatus comprising: a gas-liquid contact chamber fora gas-liquid contact of the gas with the treatment liquid, a storagechamber, located over the gas-liquid contact chamber, for storing thetreatment liquid, and a treatment liquid supplying unit for supplyingthe treatment liquid stored in the storage chamber to an inside of thegas-liquid contact chamber with a gravity-drop.
 10. The gas treatmentapparatus according to claim 9, which comprises: a plurality of thegas-liquid contact chambers, and a gas passage path for passing the gasin series through the respective gas-liquid contact chambers, whereinthe treatment liquid supplying unit supplies the treatment liquid to thegas-liquid contact chamber located at least at a most downstream sidewith respect to a flowing direction of the gas flowing along the gaspassage path.
 11. The gas treatment apparatus according to claim 9,which further comprises a treatment liquid back-flow unit for returningthe treatment liquid discharged from the gas-liquid contact chamber tothe storage chamber.
 12. The gas treatment apparatus according to claim9, wherein the gas is a carbonyl-chloride-containing gas, and thetreatment liquid is an alkaline aqueous solution.